The invention disclosed herein relates generally to ultrasonic sensors, and more specifically to single transducer ultrasonic range finders including means for automatically compensating for the effects of variations in acoustic propagation speed on distance determinations.
It is well-known to detect the presence of and/or determine the distance to a target object by transmitting acoustic energy toward the object, receiving reflections of the transmitted acoustic energy from the object and computing distance based on the round trip travel time and the propagation speed of the acoustic energy. It is also well-known that the acoustic propagation speed varies significantly depending on parameters such as the temperature, humidity and pressure of the medium through which the acoustic energy travels.
A variety of methods and apparatus have been used to compensate for such propagation speed variations. Obviously, manual calibration can be performed by locating a reference target at a known distance from the transducer and calibrating circuitry associated with the transducer so that it indicates the correct distance. Automatic calibration schemes have also been devised in which one or more significant environmental variables are measured, and the measured values used to adjust parameters of the circuitry associated with the transducer. Yet other calibration schemes are known in which the travel time of acoustic energy from a transmitting transducer to a reference transducer at a known distance from the transmitting transducer, or to a reference target at a known distance from the transmitting transducer and back to a receiving transducer at a known distance from the reference target, is used to provide a travel time versus distance relationship which can be used in determining distance to a target object.
For example, U.S. Pat. No. 3,731,273 issued to W. Hunt on May 1, 1973 discloses a position locating system adapted for use on a plotting table in which a sound generated by electrical arcing at a probe mounted spark gap is detected at a pair of spaced capacitive transducers at fixed locations. The transit times of the sound to the transducers are determined by starting counters associated the transducers when arcing is initiated, as sensed by an inductively coupled coil, and stopping the counters upon reception of sound at the transducers. An additional spark gap at a known position relative to the transducers is provided for calibration purposes. The additional spark gap is caused to arc alternately with the probe mounted spark gap, and the resulting acoustic signals received at the transducers are converted to electrical signals which are supplied to a programmed general purpose computer.
As another example, U.S. Pat. No. 3,757,285 issued to R. Ferre on Sept. 4, 1973 discloses an acoustic range measuring device in which a main transducer alternately transmits pulse of acoustic energy and receives an echo of the pulse from an obstruction. A counter determines the number of clock pulses from a reference oscillator occurring in the time interval between transmission of an acoustic pulse and reception of its echo, the count being indicative of distance. One disclosed embodiment includes an auxiliary transducer located a known distance from the main transducer for receiving samples of the transmitted acoustic pulses. A sample and hold circuit and a voltage ramp generator are connected to initiate generation of a voltage ramp upon transmission of an acoustic pulse and terminate generation of the ramp upon receipt of the pulse by the auxiliary transducer. The final ramp voltage is used to control the reference oscillator to compensate for changes in acoustic velocity due to variations in the medium through which the acoustic energy travels.
Each of the previously described systems uses an additional transducer (spark gap in U.S. Pat. No. 3,731,273) for self-calibration, which is disadvantageous in at least some respects. Most obviously, the additional transducer adds to the cost, complexity and physical size of the system.
A somewhat simpler ultrasonic system is shown in U.S. Pat. No. 4,090,407 issued to C. Shuler, et al. on May 23, 1978 which discloses a water level measurement device comprising a tube having an open end intended to extend into the water and a sending and receiving transducer at the other end for transmitting ultrasonic signals toward the water and receiving reflections therefrom. The tube is fitted with an insert having an aperture therein, the insert being located at a known distance from the transducer. A portion of each transmitted ultrasonic signal is reflected back to the transducer from the insert. The distance between the transducer and the water surface is determined by comparison of the lengths of time required for reflections to return from the insert and the water surface. The patent, however, contains no specific disclosure of the form or spacing of the transmitted signals, how reflections from the insert and the water surface are distinguished from one another, or how the comparison is performed and water depth computed.
The applicants have devised a single transducer, pulse transmission, ultrasonic sensor employing a unique method and arrangement of components for achieving self-calibration for variations in acoustic propagation speed in the surrounding medium which overcomes certain disadvantages of prior art arrangements and provides for a relatively simple, inexpensive sensor capable of good distance sensing accuracy and operational flexibility.